It warns of "significant damage to the Conservative Party in the run-up to the 2015 election" if the plans enter law.

The foreign secretary said MPs would be left to "make their own decision" when they voted on the plans next week.

The Marriage (Same Sex Couples) Bill would enable same-sex couples to get married in both civil and religious ceremonies, where a religious institution had formally consented, in England and Wales.

It would also allow couples who have previously entered into civil partnerships to convert their relationship into a marriage.

'Madness'

Meanwhile, the Sunday Telegraph reports some 180 Conservative MPs, including six whips and up to four members of the cabinet, are ready to defy the prime minister's plan to legalise gay weddings.

Analysis

By Louise StewartPolitical correspondent, BBC News

Same-sex marriage is a divisive one for the Conservatives.

Many big beasts of the party, including London Mayor Boris Johnson, support it. He says they should "just whack it through".

But this haste worries many traditional supporters.

Tory MPs are being given a free vote on the proposals and it's thought three cabinet members, as well as junior ministers and party whips, are among those likely to oppose or abstain.

Grassroots members have also made clear their opposition in the letter signed by chairmen or former chairmen of Conservative Party associations.

Giving same-sex couples the right to marry in a religious setting is something David Cameron hoped would win the party broader support.

But, it seems, the issue risks splitting the party and alienating the party's core support.

The letter to Prime Minister David Cameron, signed by 23 current and recent Conservative constituency chairmen, warns about the impact of the plans on party morale and electoral appeal.

Writing "specifically out of our concerns about the growing discord within the Conservative Party over this issue", they also say "resignations from the party are beginning to multiply".

They say the decision for the vote has been made without adequate consultation of members and that the bill is being pushed through in a way they find "extremely distasteful".

"More time should be afforded to debate an issue of such gravity… and a final decision on the matter should be postponed until after the 2015 general election when the public would have had the chance to vote on a clear manifesto pledge," they write.

Delivering the letter to 10 Downing Street, Conservative councillor Ben Harris-Quinney told the BBC: "It's frankly madness to bring this matter before Parliament at this time.

"There's no mandate for it and there are so many more serious issues affecting the country as a whole."

MPs will have a free vote on the issue, meaning they will not be issued orders by party whips, but the measure is expected to get through, with most on the Labour and Lib Dem benches supporting it.

Media captionWilliam Hague: "It's for them to make their own decision"

Foreign Secretary William Hague told BBC One's Sunday Politics: "It's for them to make their own decision, as it was when there were votes on abortion laws or capital punishment.

"If we weren't debating it now, it would be an issue at the next general election."

'Wrong time'

Marriage was "a very positive institution", Mr Hague said, arguing there was "sufficient protection" in the bill for religious organisations who did not want to offer it to same-sex couples.

He said he had changed his views on the subject in the "last couple of years" and now thought it was "right in principle".

Education Secretary Michael Gove has offered his support to the proposed legalisation of same-sex marriage but told the Mail on Sunday teachers would not be disciplined for refusing to promote it.

But he faces opposition from within his constituency association in Surrey Heath.

Its chairman, Geoffrey Veero, told the BBC "well over 25" members had already resigned over the proposed introduction of gay marriage and that the number was "rising the whole time".

"I think a number of Conservative supporters and voters will sit on their hands on the issue, and that may well affect David Cameron's opportunity to get re-elected at 2015, and we think that that is a dangerous risk to take with your core supporters."

And Conservative former education minister Tim Loughton says it was the "wrong time" to introduce such legislation.

On Saturday, it emerged the government would not introduce a tax break for married couples in next month's Budget after speculation the measure would be brought in to appease Tory backbenchers opposed to gay marriage.

However, legislation is expected to be introduced before 2015 to allow couples to transfer part of their personal tax allowance to their partner.